fix: address second code review findings

- Add context cancellation check between pagination pages in GetArtistReleaseGroups
  for responsive graceful shutdown during large discography fetches.
- Eliminate double DB query on cache hit by having GetCachedReleases return
  []ExternalRelease directly instead of just a count, avoiding a redundant
  second query in SyncArtistDiscography.
- Update cache_test.go to match new GetCachedReleases return type.
- Format main.go (pre-existing whitespace issue).

internal/musicbrainz/cache.go

@@ -9,11 +9,11 @@ import (
 
 // GetCachedReleases queries the external_releases table for entries
 // belonging to the given artist that were cached within the specified TTL.
-// It returns the count of cached entries and any error encountered.
-func GetCachedReleases(db *database.DB, artistID string, ttl time.Duration) (int, error) {
+// It returns the cached releases and any error encountered.
+func GetCachedReleases(db *database.DB, artistID string, ttl time.Duration) ([]database.ExternalRelease, error) {
 	releases, err := database.GetExternalReleasesByArtistWithCache(db, artistID, ttl)
 	if err != nil {
-		return 0, fmt.Errorf("get cached releases: %w", err)
+		return nil, fmt.Errorf("get cached releases: %w", err)
 	}
-	return len(releases), nil
+	return releases, nil
 }